1. Field of the Invention
The present invention relates to an optical wavelength filter.
2. Description of Related Art
Conventionally, an optical wavelength filter, which selectively outputs light of a specific wavelength from light entered into a Mach-Zehnder optical element having a diffraction grating, was utilized. In Document I (Optical Fiber Communication (OFC""99) TuN 3-1), an optical add-drop element is disclosed as an example thereof.
The optical add-drop element disclosed in Document I has either one or more optical couplers and a diffraction grating in each of a plurality of parallel optical waveguides. The diffraction grating either, for example, is disposed inside one optical coupler like the Bragg reflecting coupler shown in FIG. 4 of Document I, or, for example, is disposed on two arms between two optical couplers as in the bandpass filter having a Michelson-Bragg diffraction grating shown in FIG. 1 of Document I.
With such an optical add-drop element, by making light of a specific wavelength incident upon the diffraction grating, it is possible either to drop (to make wavelength division demultiplexing) light of a specific wavelength from the incident light, or to add (to make wavelength division multiplexing) light of a specific wavelength to the incident light.
However, in the optical add-drop element of Document I, the problem is that since the light propagating in the optical coupler ordinarily becomes a propagation light of a high-order mode comprising a plurality of specific (or intrinsic) modes, this gives rise to differences in the propagation constant between the specific modes, and the wavelengths reflected on the diffraction grating will differ for each mode.
That is, when wavelength division multiplexing light (WDM light or WDM optical signal) is made incident on the optical add-drop element, for example, whereas ideally single peak output light having a peak in only one specific wavelength should be selectively outputted, because the reflected wavelength selected in the grating differs for each mode, double peak output light having peaks in a plurality of barely separate wavelengths is outputted.
Further, as disclosed in Document II (Japanese Patent Publication No. 7-89183 (Japanese Patent No. 2053167)), if an optical coupler is constituted such that only one specific mode is excited, this kind of problem does not occur, but, in this case, the optical coupler must be constituted from an asymmetrical branching waveguide.
However, in this asymmetrical branching waveguide, since it is necessary to reduce the branching angle, for example, to around 0.1 xcexcrad, and to quasi-statically change the structure relative to the direction of propagation of the light, the length of the element inevitably becomes longer, and as a result thereof, it was impossible to realize a more compact optical wavelength filter.
Therefore, an object of the present invention is to provide an optical wavelength filter, which is capable of suppressing the generation of double peaks caused by differences in the propagation constant between specific modes in an optical coupler.
Another object of the present invention is to provide an optical wavelength filter which, more preferably, is capable of suppressing the generation of double peaks without increasing the length of the filter.
Accordingly, an optical wavelength filter of the present invention comprises a first and second optical coupler and a mode converter. Each of the optical couplers propagate wavelength division multiplexing (WDM) light comprising a plurality of light components with different wavelengths, excite multiple modes of light including high-order mode light for each wavelength of light of the WDM light, and provide a predetermined phase difference between different modes of the multiple modes of light. The mode converter is arranged between the first and second optical coupler, and performs mode conversion between modes of different orders of a specific wavelength selected upon request from among the multiple modes of light excited by the optical couplers.
According to this constitution, because mode conversion is performed between modes of different orders of a specific wavelength selected as desired from among the multiple modes of light excited by the optical couplers, it is possible to adjust the phase difference between modes of light belonging to a specific wavelength of light independently from the phase difference between modes of light belonging to another wavelength of light. Accordingly, it is possible to output only light of a specific wavelength from an output or exit port separate from that for light of another wavelength. Since mode conversion is performed between modes of light belonging to a specific wavelength, it is possible to suppress the generation of double peaks that is caused by the difference in propagation constant for each mode of light in a specific wavelength.
Note that mode conversion in a mode converter signifies mode conversion between at the least modes of different orders. However, in addition thereto, there can also be cases in which mode conversion is performed for reflection and so forth.
Further, in an embodiment of the present invention, preferably a first optical coupler has a first input port for inputting WDM light thereto, and a first optical coupling region for exciting the multiple modes of light of the WDM light inputted from the first input port thereof, and in the optical coupling region, a phase difference of substantially xc2x1xcfx80/2 is provided between the modes of light belonging to each wavelength of light, and a mode converter performs the mode conversion described hereinabove while generally maintaining the phase difference between the modes of light outputted from the first optical coupler, and a second optical coupler has a second optical coupling region for enabling the propagation of multiple modes of light, and a first output port and a second output port, and in this second optical coupling region, once again a phase difference of substantially xc2x1xcfx80/2 is provided between the modes of light belonging to each wavelength of light outputted from the mode converter, and in accordance therewith, the light of a specific wavelength and other light can be outputted from either the first or second output ports, which differ from one another.
By so doing, from among the modes of light constituting WDM light inputted to this optical wavelength filter, a phase difference of either xcfx80 or xe2x88x92xcfx80 can be provided between the modes of light belonging to a specific wavelength, and a phase difference of either xe2x88x92xcfx80 or xcfx80 (double signs in same order) can be provided between the modes of light belonging to other wavelengths. That is, it is enabled to output the light of a specific wavelength and the light of other wavelengths from different output ports.
Furthermore, in the first optical coupler, second optical coupler and mode converter, a phase difference, which is either granted or maintained may be considered in a range from xe2x88x92xcfx80 to xcfx80 in accordance with periodicity of 2xcfx80 for each mode of light.
Further, according to an optical wavelength filter of the present invention, because a first and second optical coupler can be formed by a coupler having a symmetrical branching structure, there is no need to utilize such an asymmetrical branching waveguide as disclosed in Document II, and as a result thereof, the element length of the optical wavelength filter can be shortened.
Further, in another preferable embodiment of the present invention, multiple modes of light excited by the first and second optical couplers may be made zero-order mode light and first-order mode light, and in that case, mode conversion in the mode converter can be performed between a zero-order mode light and a first-order mode light each constituting a specific wavelength of light. That is, an optical wavelength filter may be constituted as a 2-mode system, which utilizes two modes of light.
Further, in another embodiment of the present invention, it is preferable that multiple modes of light excited by the first and second optical couplers may be made zero-order mode light, first-order mode light and second-order mode light, and, in that case, mode conversion in the mode converter may be performed between a zero-order mode light and a second-order mode light, both constituting (or belonging to) a specific wavelength of light.
In a still another embodiment of the present invention, it is preferable that multiple modes of light excited by the first and second optical couplers may be zero-order mode light, first-order mode light, second-order mode light and third-order mode light, and in this case, mode conversion in the mode converter may be performed between either one of a zero-order mode light and a second-order mode light, or a first-order mode light and a third-order mode light, constituting (or belonging to) a specific wavelength light.
Thus, when constituted as a 3-mode system comprising zero- through second-order modes of light, or as a 4-mode system comprising zero- through third-order modes of light, or as a mode system of orders thereabove, since crosstalk light becomes apt to form a radiating mode in the output-side optical coupler, it is possible to reduce unnecessary crosstalk light.
Typically, a mode converter has a multimode waveguide, and a plurality of rows of diffraction gratings. The diffraction gratings are provided in the multimode waveguide so as to form a periodic structure along the optical wave guiding direction in the multimode waveguide, and such that the rows extend in parallel while being mutually staggered by a half period.
A mode converter may be constituted by a multimode waveguide as mentioned above. Accordingly, a mode converter may also be constituted from a plurality of single-mode waveguides as will be described hereinbelow. Specifically, a mode converter, for example, may also have a plurality of single-mode waveguides arranged in parallel, and a plurality of rows of diffraction gratings. The diffraction gratings are provided in the respective single-mode waveguides so as to form a periodic structure along the optical wave guiding direction, and such that the rows extend in parallel while being mutually staggered by a half period.